The basic components of a chromatographic instrument (hereinafter, chromatograph) include an injection port for introducing a sample to be examined into a stream of carrier medium, a separation column attached to the injection port, and a temperature-controlled zone in which the column is mounted. Typically, the column is constructed in the form of a helix of tubing containing a stationary phase that causes some of the constituents of the sample to elute at different times. The output end of the separation column is attached to a detector for producing a signal indicative of the concentration of the constituents being eluted.
With reference to FIG. 1, a portion of a typical chromatograph is illustrated, wherein the temperature-controlled zone is contained within an oven 10. The oven 10 typically comprises a thermally insulated oven housing 20 having an opening for access to the oven interior 18. An oven heater includes a controlled heating element and a motor-driven stirring fan (not shown) situated behind a perforated section 21 in a shroud 22. The injection port and detector (not shown) are attached to respective pneumatic fittings 12, 14 on the oven 10. The separation column 16 is typically provided in the form of a coil of tubing mounted on a basket 17 positioned within the oven interior 18 such that the inlet and outlet ends of the separation column 16 may be attached to the pneumatic fittings 12, 14.
In analyzing most samples, the heating element is controlled so as to vary the temperature of the oven according to a predetermined temperature profile. It is also known to operate the stirring fan to draw oven cavity air into the heater through the perforated section 21 and force heated air from the oven heater through the periphery of the shroud 22 and into the oven cavity.
The stirring fan mixes the heated air from the heating element with oven cavity air, with the goal of creating a uniform thermal profile. The stirring fan is thus employed in an attempt to circulate an air "bath" throughout the oven cavity so as to minimize thermal gradients that could adversely affect the performance of the chemical process occurring within the column. Conventional stirring fans typically employ a mixed flow impeller to create a vortex airflow. The resulting air bath develops a high pressure region of newly heated air at the periphery of the shroud and a low pressure region in the center of the oven interior. This technique is intended to maximize the time for air mixing and to employ the oven walls as a thermal damper to minimize thermal gradients.